Galvanic Corrosion Protection for DHW Side-Arm Heat Exchanger

[timberbuilt]

I've got my DHW plumbed in to be heated with a side-arm heat exchanger. Originally I did not circulate the DHW side but found it ineffective so added a circulating pump on the DWH side (boiler side also circulates). I've experienced corrosion issues on the DHW side. Yes, I do have dissimilar metals in contact, mainly copper, brass, stainless steel and black steel as well as whatever I'm contacting at the DWH tank heater. I've researched the galvanic corrosion issue a bit although there seems to be a wide variety of opinion on the matter. It doesn't appear there is a direct commercial product available for the problem. Does anyone have any experience with similar corrosion issues? What about experience utilizing a sacrificial anode (e.g.corrosion grenade or similar)? I do believe I've lost an electric DHW heater to the problem...

 

[maple1]

What exactly corroded? The tank? It should have had an annode in it already if so, and if it is a standard electric DHW heater. Age? Maybe regular replacement of it?

 

[jrod770]

Black steel should never be used for potable water

 

[GeHmTS]

Do you have pictures of the setup? A picture of the corrosion would also be helpful.

 

[timberbuilt]

Per the question about what corroded on the hot water tank, it was the tank wall. I wish I had pictures but I was in a big hurry to get the hot water back up and running at the time so didn't get pics. Hot water tank was the same age as the full hydronic system, maybe 4 y.o. I had the plumbing into the DHW tank a little funky with the side-arm going in through the pressure relief hole on the top instead of on the cold water side. I've now installed the side-arm on the cold water side, which has plastic threading, which so far seems to be preventing corrosion there.

I know, I know the obvious answer is to try replacing the black steel female-female adapters to copper. Problem is the side-arm is buried back behind the DHW and pressure tanks AND the adapters take some awkward heft with a couple of large pipe wrench to detach and attach so it's a fairly cumbersome job at this point. I also know, I know I shouldn't have used the black steel but I live in the sticks and at the time the hotwater was down and the wife and kids were getting cranky and stinky, respectively of course, and the black steel was my only option at the time.

Easy answer for me would be to try a sacrificial anode, although it may come down to that I need to bite the bullet and replace the black steel.

Anyways, any experience with similar would be interesting and grateful. CP seems to be a debatable subject and I'm having a hard time making sense of the right and wrongs. Apparently I've got quite a lot of current being created from all of the water flow through the heat exchange, which makes sense as the DHW is flowing one way and the boiler water the other. I'm also curios if the current that causes the galvanic corrosion travels in the water and I could see similar issues in other places or if it is just a matter of dissimilar metals in contact. So far the hydronic system seems to not be effected, but it is a closed system which I think makes sense.

 

[maple1]

A 4 year old electric hot water heater should have had an anode in it, to start with. Any I have seen, come that way.

So I guess my question would be - what was the condition of the anode rod in the tank that corroded?

And if you replaced it with a new tank (I think you did but not sure?), it should have a rod in it already also. I have read some recommendations that the rod should be replaced every couple of years, and checked annually. That might be in extreme conditions, I don't know - but I think your tank corrosion would be more from the condition of your water than the fact you have a sidearm hooked up and the current flow. I have a sidearm, in series with a FPHX, with both sides pumped, no signs of issues after 3 years - although I haven't checked the anode (likely should have before this). I also plumbed the top of that circuit into the pressure relief tapping as you did at first.

Another factor aside from the couple of black iron/steel fittings you have, is what exactly do you have for a pump on the DHW side? I used a B&G Ecocirc that was made for DHW recirculation - no steel in it. If you used an ordinary heating system circ, that is another possible source of corrosion.

 

[timberbuilt]

Good points Maple. The old DHW was a new install and you are right it should have and probably did have an anode, so the corrosion was bad enough it must have used it up (Or totally separate issue). The circ pump on the DHW side is a proper stainless Taco 008 tied in with copper and brass AND separated from the sidearm with pex. I wonder if a poor ground on it would be a poor ground on the pump? Something else that I haven't considered yet is the possibility that what I'm seeing as corrosion is in fact water leaking and depositing precipitate or scale. That might help explain what happened to the first tank? The threaded connections on the sidearm are a SOB to get tight because there's nothing but the flimsy stainless side arm housing to put backup on.

 

[Medman]

pics definitely show leaking. I used a black male-male nipple on the DHW side of my setup for the same reason as you - it was all I could get. It plugged completely inside within a year, and the threads rotted through and started leaking. Even though its a real pain, you have to get that black pipe out of the DHW side of the system. Before you have a really big mess.